Ancient fish
teeth are taking a bite out of an old conundrum about how Antarctica became
the frigid continent that it is today. The teeth suggest an early start to key
oceanic processes that drove the climatic shift.

This fossil fish tooth, only a few tenths
of a millimeter long, and many more like it provide evidence that Pacific and
Atlantic waters mixed earlier than scientists previously thought. Photo is by
Howie Scher.

Antarctica once flourished with boreal forests and a temperate climate. Then,
about 34 million years ago the climate changed and quickly turned the continent
into a huge ice sheet (see Geotimes,
March 2006).

The culprit for the sudden change, according to some paleoclimatologists, was
the onset of the Antarctic Circumpolar Current (ACC)  a cold current that
encircles Antarctica and isolates the continent from warm subtropical waters.
For the ACC to form, however, there needed to be an open passageway around Antarctica
without restrictions from neighboring continents. As long as theres
no barrier there, the current can just spin up and go around, says Ellen
Martin, a paleoclimatologist at the University of Florida in Gainesville.

Although the opening of the Tasmanian Gateway between Australia and Antarctica
is thought to have spurred formation of the current, another ocean opening event
 of the Drake Passage between South America and Antarctica  could
have kick-started the necessary oceanic processes. Geoscientists have long debated
the timing of the opening of the Drake Passage: Previous estimates based on
continental plate spreading have placed the event anywhere from 20 million to
45 million years ago, lending some uncertainty about the events suspected
effect on climate.

Now, Martin and Howie Scher of the University of Rochester in New York have
used isotopic data to zero in on a period 41 million years ago for the opening
of the Drake Passage. The new date places it before the opening of the Tasmanian
Gateway, but in time to make the Drake Passage a key player in the cooling down
of Antarctica, they report in the April 12 Science.

To date the opening, Martins team analyzed micrometer-sized fossil fish
teeth extracted from core samples pulled from the southern Atlantic Ocean. Fish
teeth absorb the element neodymium, which rivers carry from nearby rocks into
Earths oceans. Neodymium that comes from young volcanic rocks into the
Pacific Ocean, however, has a different isotopic ratio than the same element
that comes from older rocks into the Atlantic Ocean.

Measurements of neodymium in teeth from the Atlantic turned up a large change
in the elements ratio at about 41 million years ago. The change reflects
water from the Pacific mingling with water in the Atlantic, implying that the
Drake Passage had opened, Martin says. By showing that we could see the
Pacific water coming through, were adding a little more credence to the
numbers, she says.

While the shift indicates that the Drake Passage opened by that time, it does
not speak to the depth of the passage nor how much water was mixing. Its
a little harder to get a handle on that, Martin says.

Lawrence Lawver, a marine geophysicist at the University of Texas in Austin,
agrees that continental plate movements may have allowed a shallow Drake Passage
to have opened by 41 million years ago, but that a deep opening by that time
is highly unlikely.

Still, Martin and colleagues suggest that even a shallow opening of the Drake
Passage would be enough to start nutrient upwelling in the oceans that would
have drawn down carbon dioxide from the atmosphere. That change would have created
a cooling that contributed to a corresponding momentary period of Antarctic
glaciation prior to the long-term event that started 34 million years ago, Martin
says.

Shallow or deep, the timing of the Drake Passage opening places it well before
the Tasmanian Gateway opening, which scientists think deepened between 35.5
million and 33.3 million years ago. The Tasmanian opening likely established
an open path for the ACC to flow, thanks in part to upwelling processes that
started earlier with the Drake Passage opening. What our date does,
Martin says, is show that the Drake Passage is old enough now that it
could have played a key role in helping make Antarctica cold.